1. Field of the Invention
This invention relates to a variable magnification copying apparatus of the slit exposure type in which variable magnification copying is effected with a copy medium maintained at a predetermined velocity but with the velocity of optical scanning means varied.
2. Description of the Prior Art
In order to obtain two or more steps of copy magnification in a variable magnification copying apparatus of the slit exposure type, there is usually adopted a method of changing the velocity of the optical scanning means while maintaining the copy medium at a predetermined velocity. According to this method, variable magnification copying can be easily accomplished by using entirely the same factors as those for the 1.times. magnification copying in the processes such as charging and development of the photosensitive medium and in all the other driving and conveying mechanisms than the optical system. The scanning of an image original by the optical system is usually carried out in two alternative ways, one of which uses a movable optical system in which an illumination system, mirrors and the like are movable and the other uses an original carriage movable with respect to the optical system. However, the optical scanning means mentioned herein means one of the optical system and the original carriage which is movable. Usually, optical scanning means must be moved at a constant velocity as long as it optically scans an image original in the form of a slit. Therefore, a suitable preliminary running range is provided on the way to one edge of the image original so that before the optical scanning means illuminates said one edge of the image original, the optical scanning means may assume a constant velocity and a light source for illumination may assume its steady state condition.
Where variable magnification copying is effected by using the above-described method and for example, if the velocity of a copy medium is V, the velocity of the optical scanning means during the 1.times. magnification copying is of course equal to V, but the velocity of the scanning means during the N.times. magnification copying is V/N. Consequently, the time required for the optical scanning means to scan over the aforementioned preliminary running range differs from that required for the 1.times. magnification copying.
Thus, if copying is effected with the velocity of the optical scanning means alone varied but with the other conditions maintained identical to those for the 1.times. magnification copying, the copy image formed on the copy medium may lack a portion corresponding to the one edge of the image original or conversely, a large blank or unimaged area will exist on the copy medium before the one edge of the image original is copied, and this will sometimes result in an inconvenience that the other edge portion of the image original is left uncopied.
To correct the misregistration between the image original and the copy medium resulting from such change in magnification during the variable magnification copying, the following methods would occur to mind.
(1) With respect to the copy medium effecting a predetermined movement, the timing for starting the scanning of the optical scanning means is changed in accordance with each magnification desired;
(2) With respect to the copy medium effecting a predetermined movement, the preliminary running range for the optical scanning means is varied;
(3) With respect to the optical scanning means effecting a predetermined movement, the timing for starting the scanning of the copy medium is changed in accordance with each magnification desired;
(4) To detect the position of the copy medium and control the timing for starting the scanning of the optical scanning means; and
(5) To change the position of the image original on the original carriage in accordance with each magnification desired.
However, the method mentioned under item (1) above would require a number of timers corresponding to the steps of magnification to be provided for controlling the optical scanning means, and these timers would have to be adjusted for each magnification desired.
Also, since the optical scanning means has a certain degree of mass, there would be created a time loss from when the optical scanning means is subjected to a drive until it assumes a constant velocity of movement, namely, till the rising of the scanning means. Where the drive means is a motor or the like, the time loss would be varied by the state of the power source, temperature and frequency of use, and where the drive means is a magnet clutch or the like, the time loss would be varied by the state of the power source and the frequency of use, and it would be nearly impossible to achieve the aforementioned adjustment in the cases including these.
Further, in order to prevent the home position, namely, the preliminary running range, of the scanning means from being varied for each scanning cycle, a mechanism would be required for holding the scanning means at its home position with high accuracy.
The method mentioned under item (2) above would involve, in addition to the above-noted problems, the necessity of providing a complicated mechanism for driving the scanning means to a predetermined position before it starts scanning each time the copying magnification is changed.
The method mentioned under item (3) above would suffer from the drawbacks similar to those noted with respect to the item (1).
The method (4) is almost similar to the method (1) and instead of the timers, a number of copy medium detecting means corresponding to the steps of copy magnification would have to be adjusted.
The method (5) would require the operator of the apparatus to change the position of the image original in accordance with each magnification desired, and this would most undesirably lead to complicated manipulation and accordingly malfunctioning of the apparatus.